1. Field of the Invention
The invention relates to a metallization for a housing technology, for example for surface acoustic wave (SAW) components for use in strong magnetic fields, for example in magnetic resonance imaging (MRI) systems, and a hermetically sealed nonmagnetic cavity housing.
2. Description of the Related Art
To attain high resolution, MRI systems operate with a plurality of magnetic induction coils, wherein the signals for each coil are typically processed via a separate electronic circuit. Because this electronic circuit is typically operated inside an MRI system, all employed components must be completely nonmagnetic, i.e., must not include ferromagnetic materials, so as to prevent field disturbances.
Hermetically sealed housings are required for obtaining components with long-term stability, so that technically simple solutions, for example based on printed circuit board material, would not be appropriate due to reliability concerns. The invention relates to the improvement of the established housing technology based on multilayer ceramics, wherein the variant established in the industry does not achieve the desired objective because of the nickel content in the ceramic coating and because of the iron, nickel and cobalt content (Kovar, FeNi42, etc.) in the welded or soldered cover.
MRI systems require frequency filtering downstream of the signal-generating induction coils and immediately before a first signal processing. Presently, this filtering task is mainly performed by discrete filters constructed from nonmagnetic coils and capacitors. Disadvantageously, these filters have as a discrete design and require considerable space, require manual frequency tuning, resulting in high labor costs and a low edge steepness of the filters. The use of surface acoustic wave (SAW) filters significantly overcomes the aforementioned deficiencies; however, established housing technologies are mostly unsuitable because of their magnetic property.
Because of the required long-term stability of the filters, any contamination and aging of the micro-acoustic active structure disposed on the surface of the piezoelectric crystal must be prevented, thus requiring hermetically sealed housings. These housings are preferably implemented in the SAW-technology, primarily for filters having a relatively large size, on the basis of multilayer metal or ceramic housings, wherein both technologies normally use ferromagnetic materials. Whereas the former consists of iron alloys, ceramic housings typically use nickel-containing metallization systems for internal and external conductor structures, which have impermissibly strong magnetic properties. Still stronger magnetic properties furthermore result from the nickel-coated covers made of iron-nickel-cobalt (Kovar) alloys which are used for closure and are either soldered or welded with the housing—in the latter case on a sealing ring, typically made of Kovar, placed underneath.
Newer (e.g., Chip Scale Package or CSP) housing technologies for SAW components predominantly focus on smaller sizes and therefore primarily on high-frequency filters, for example for mobile radio applications.
Due to the typically relatively low filter center frequencies below or not significantly above 100 MHz required, for example, for MRI applications, these technologies are not appropriate for the described application for several reasons.
U.S. Pat. No. 7,253,029 B2 describes a technology, wherein in order to avoid magnetic properties, the adhesive nickel layer normally used with this technology and deposited directly onto the tungsten layer is replaced with a palladium layer having similar chemical, but nonmagnetic properties. To obtain stable properties in processes used for building, for example, wire bonds and soldering that are comparable with conventional nickel layers—typically having thicknesses between 2 and 10 μm—, a relatively thick palladium layer is required which is disadvantageous for cost-sensitive applications due to the high material price of the noble metal. Conversely, palladium layer thicknesses of, for example, 1 μm having manageable costs do not represent a suitable foundation for achieving mechanically stable wire bond connections, which are essential for providing electrical contact between the housing and chip.
U.S. Pat. No. 4,941,582 discloses a method for producing a layer which is stable against soldering for Low Temperature Cofired Ceramic materials (so-called LTCC materials) with copper-based metallizations having processing temperatures below 1100° C., typically below 1000° C. However, Low Temperature Cofired Ceramic materials (so-called LTCC materials, for example Al2O3 ceramic) require firing temperatures of about 1500-1700° C., which precludes the use of—low-resistance, but also low-melting—copper (melting point of ca. 1085° C.) directly on the ceramic, thus necessitating the use of materials having a high melting point, such as tungsten (melting point of ca. 3422° C.) or molybdenum (melting point of ca. 2623° C.), which are disadvantageously also relatively poor electrical conductors. In general, LTCC materials are always used when, in addition to providing only a housing, additional passive components—typically capacitors, inductors or delay lines—are to be integrated into the housing in form of ceramic intermediate layers, as described in U.S. Pat. No. 4,941,582. Disadvantageously, LTCC primarily have higher costs and lower mechanical stability compared to HTCC. Palladium and nickel are explicitly described in U.S. Pat. No. 4,941,582 as a separation layer and hence as a diffusion barrier between Cu and Au.